Field of the Invention
The invention relates to nanocomposite materials and methods of making thereof.
Related Art
The market for aerospace composites is expected to exceed $50 billion in 2014. These materials make up more than 50% of the latest generation of aircraft because of their advantageous low density, excellent combination of stiffness and toughness, and resistance to corrosion when compared to conventional aluminum alloys (1). Unfortunately, these materials, which took decades to become accepted by the aerospace industry, are running into limitations inherent to their composition and production methods, such as poor high-temperature tolerance and expensive, complicated processing. Nanocomposites present a potential route to both improved properties and simpler processing, as well as access to new and multiple functionalities (2), and are the key to unlocking the next revolution in aerospace materials technology.
A nanocomposite is a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nm, or structures having nano-scale repeat distances between the different phases that make up the material. In mechanical terms, nanocomposites differ from conventional composites due to the exceptionally high surface to volume ratio of the reinforcing phase and/or its exceptionally high aspect ratio. The mechanical, electrical, thermal, optical, electrochemical, and catalytic properties of the nanocomposite will differ markedly from that of the component materials. This “emergent” (the whole being greater than the sum of the parts) functionality property places nanocomposites in the optimal design space to provide a solution to both the “materials-by-design” and “multifunctionality” requirements for the next generation of synthetic engineering materials (3,4).
Synthetic nanocomposites are an important research target in materials science. The use of nanoparticle-rich materials long predates the understanding of the physical and chemical nature of these materials. From the mid-1950s nanoscale organo-clays have been used to control flow of polymer solutions (e.g. as paint viscosifiers) or the constitution of gels (e.g. as a thickening substance in cosmetics, keeping the preparations in homogeneous form). By the 1970s polymer-clay nanocomposites were commercially viable commodity materials. This successful system is nonetheless limited to low nano-object volume fractions, due to synthesis and processing obstacles that have thus far prevented the full realization of the potential engineering benefits and game-changing technologies presented by nanocomposites.
Nanocomposites are nature's universal materials-by-design solution. Biological nanocomposites may possess >95% inorganic volume fraction in <5% polymer matrix, as in the case of abalone nacre, giving this material both strength and toughness (5). Biological nanocomposites may also possess zero inorganic component, as in the case of spider dragline silk, which is among the toughest and strongest materials known, as a result of nanoscale reinforcement by organic crystallites. The modular design of biological nanocomposites enables a wide range of mechanical properties to be obtained from the same starting components and manufacturing process (6). For example, stiff and tough bone and strong and extensible tendon vary only in their degree of inorganic nanoscale reinforcement. The dynamic nature of the self-assembly processes makes the resulting materials adaptive and highly tolerant of minor manufacturing flaws. Critically, the modular design and universal process capability of biological-nanocomposites enables the facile production of mechanical property gradients, for example at the interface of bone and tendon, minimizing interfacial stresses, a key failure mechanism in synthetic materials. An extreme example of this effect is seen in the squid beak, the hardest natural substance at its edge, which must bond to the soft and flexible squid body (7). Finally, biological nanocomposites possess emergent multifunctionality, such as the ability to change mechanical properties in response to environmental stimuli, as in the case of the Sea Cucumber dermis (8).
The ability to spontaneously heal injury is another key emergent functionality found in biological nanocomposites that increases the survivability and lifetime of most plants and animals. In sharp contrast, synthetic materials usually fail after damage or fracture.
For decades, scientists and engineers have dreamed of developing self-healing materials to improve the safety, lifetime, energy efficiency, and environmental impact of manmade materials (9). The first successful demonstration of a spontaneously self-healing (requiring no external input or trigger beyond the damage itself) involves micro-encapsulated healing agents and catalysts embedded in a conventional polymer matrix (10). While this approach is very effective for the initial damage-healing cycle, further damage in an already healed region is not reversible due to prior consumption of the healing agents. For most new designs, external energy is required to achieve healing. For example, thermally reversible covalent bonds or non-covalent supramolecular linkages were introduced into polymers, which upon heating can reversibly rupture and reform to afford self-healing (11). Recently, a metallo-supramolecular polymer was shown to be thermally mendable by converting photo energy into localized heat. In this microphase-separated system, the metal complex healing motifs reside in the crystalline hard domain, which requires thermal energy to reversibly dissociate in order to heal (12). For many applications, however, autonomic healing without any external stimulus is desirable. Toward this goal, an elegant dynamic supramolecular approach was developed to achieve a self-healing rubber by employing multivalent hydrogen bonds, which though individually weak, collectively form a load-bearing network that is dynamic at room temperature, allowing automatic healing of damage. However, the lack of molecular/nano-level structure control severely limits both mechanical properties and processing of this system (13).